Feature/ja 117 activation emails

[skrawus]

I added sending activation emails. Still in progress.

[Zjyslav]

• Move activation link to configuration
• Move activation code generation to UserAuthenticationService (return the activation code as part of result of RegisterUser, instead of putting it in as a parameter)

[Zjyslav]

• Move all logic touching db to UserService.

I'm thinking about putting the IsUserActive logic as part of LogInAsync. It already recevices the user from UserService, so it could return not just a bool, but a result object containing the code and we wouldn't need to call the db twice.
Now that I think of it, we log the user in, then we check if the account is active and if it isn't, they are still logged in (even though they are shown a failure message). I think this check should be done as part of logging in and we could return a result with an enum or something telling the controller what was the reason of failure (wrong data, inactive account, etc.).

[Zjyslav]

I approve, but please resolve merge conflicts.